Add missing atomic/nonatomic property declaration in header.

First, a property was synthesized atomic by default.

For example,
```
@property (assign) MBProgressHUDMode mode;
```

was equal to
```
@property (assign, atomic) MBProgressHUDMode mode;
```

So the atomicity arguments should be added to the statements.


Second, I changed some atomic arguments to nonatomic. 

Its not thread-safe if you access a atomic property's ivar directly. So I assume them should be nonatomic. eg. `mode`, `minSize`, `minSize`, `delegate`.

And I think the properties relate to UI also should be nonatomic. eg. `customView`, `opacity`.